Abstract:

Quartz in mudstones of the Mowry Shale (Cretaceous, Rocky Mountain region) takes several forms: extrabasinal detrital silt (some with transported overgrowths), replacement of skeletal debris, minor overgrowth on detrital quartz and replaced radiolaria, pore-filling in the intragranular pores of allochems, and authigenic microquartz that is dispersed through the clay-size matrix. The matrix-dispersed microquartz has CL character similar to the overgrowths with which it is intergrown, supporting the interpretation that it is authigenic and pore-filling. Radiolaria are widely observed in these mudrocks but are poorly preserved by partial mineral replacement (quartz, dolomite, calcite, pyrite). We interpret dissolved radiolaria, and possibly, now-cryptic diatoms, as the likely source of silica for the overgrowths and microquartz, through a reaction pathway that entailed the formation of opal-CT lepispheres that subsequently recrystallized to microquartz. A newly devised variation of the IGV (intergranular volume) concept, a parameter IGVm, is applied for partitioning the causes of porosity decline in mudstones. All of the samples examined contain authigenic microquartz, which averages around 40% of the rock volume. The large volume of microquartz, together with the volumes of surviving intergranular pores and pore-filling solid hydrocarbon, constrain the porosity to around 50% at the time of initial lepisphere formation. Despite the exceptional abundance of cement, overall porosity decline is compaction-dominated. The presence of significant volumes of cement has implications for mechanical rock properties in the Mowry Shale, which is expected to manifest brittle behavior where cements are most prominently developed.